It is generally understood that the inhibition of growth of root apices is the initial effect caused by aluminium (Al) toxicity. The correlation between impaired H+-fluxes across the plasma membrane (PM) and Al-induced growth inhibition, Al accumulation and callose formation in root apices of squash (Cucurbita pepo L. cv. Tetsukabuto) is reported here. The root inhibition was dependent on Al concentration, and the duration of exposure, with the damage occurring preferentially in regions with high Al accumulation and callose formation. Using the fluorescent Al indicator (Morin), Al was localized in the cell walls of the root-tip cells after 3 h and in the whole root-tip cells after 6 h of the Al treatment (50 muM). The inhibition of H+-pumping rate in the highly purified PM vesicles obtained from the Al-treated apical root portions (1 cm) coincided with the inhibition of root growth under Al stress. Furthermore, H+-ATPase activity of PM vesicles prepared from the control root apices was strongly inhibited by Al in vitro in a dose-dependent manner. Approximately 50% inhibition was observed when PM vesicles were preincubated at Al concentration as low as 10 muM followed by the enzyme assay in the medium without Al. Using the pH indicator (bromocresol purple), it is shown that surface pH of the control (0 Al) root apices was strongly alkalized from the starting pH of 4.5 in a time-dependent manner. By contrast, the surface pH changed only slightly in the Al-treated root apices. The changes in surface pH mediated by altered dynamics of H+ efflux and influx across the root tip PM play an important role in root growth as affected by Al.